Gel chromatography methods for fractionating systems having varying molecular weights are well known.
The method finds widespread utility in the area of fractionating polymer systems of diverse molecular weight. A bed or mass of solvent swollen polymer (hereinafter referred to as the "gel") is provided. Normally, the gel comprises a solvent swollen polymer, e.g., styragel, wherein the gel consists mainly of an uncharged water-insoluble polymer which is inert with regard to the substances to be separated and capable of swelling in an organic solvent medium. The gel is also capable of selectively and/or preferentially separating substances from the solution to be fractionated so that substances with different molecular weights or sizes are distributed differently between the gel and the surrounding solution owing to their different ability to penetrate into the gel, which is in turn dependent upon their molecular weights or sizes.
An organic solvent solution of the system to be fractionated is passed through the mass or bed of gel.
The medium in which the gel is swollen is displaced from the gel bed as the solution of material to be fractionated is fed thereto. The separating capacity of the gel is dependent on the molecular weight or size of the substances to be separated and on the size of the pores or meshes in the three-dimensional network of the gel itself. The higher molecular weight or larger molecules, incapable of penetrating the gel, remain in solution and pass through the interstices of the gel without penetration. The smaller or lower molecular weight molecules penetrate into the gel, dependent upon the size of the pores or meshes therein, and are temporarily taken up thereby. Stated differently, the larger or higher molecular weight molecules remain outside the gel in the solvent in which they were introduced to the gel bed. The smaller or lower molecular weight molecules are temporarily trapped in the interior solvent with which the gel was swollen, inside the porous gel. It is to be understood that the separation is based upon a molecular sieve-type mechanism rather than on a sorption of the fraction into the gel.
Given a gel bed of a certain height, the larger or higher molecular weight molecules will tend to be concentrated at the bottom, carried down by the solvent in which they were introduced. The lighter or smaller molecules will tend to penetrate into the gel at a higher level, thereby achieving a chromatographic fractionation. Obviously, the broader the pore size distribution of the gel bed, the broader the range of molecular weight fractions that can be separated.
Separation can be achieved by feeding an elution liquid into the bed to displace the solution introduced into the bed and the internal solvent with which the gel was swollen. The larger or higher molecular weight particles will be eluted first since they have not fully penetrated the gel. The smaller molecular weight molecules temporarily trapped within the gel will be eluted later, thereby achieving substantially complete separation of the diverse molecular weight fractions.
The above-described gel chromatography method has proved to be very useful for the fractionation of polymers. The method has been used to determine the molecular weight distribution of various polymer systems or for the preparation of fractions of polymer systems with well-defined molecular weight distributions. Because of the high resolving power, speed and possibility of a high degree of automation, gel permeation chromatography is the most promising method for the fractionation of polymers both on an analytical scale and on a preparative scale.
Although the gel permeation chromatography method has been applied to solutions of polymers, no satisfactory method has heretofore been proposed for the fractionation of so-called ionic polymers without altering the structure of the polymer by an ion exchange or other chemical or physical process prior to the fractionation.
Moreover, no system has been heretofore proposed which enables the use of aqueous solvents or elution liquids on a practical scale.
It is an object of the invention to provide a method for the fractionation of the diverse molecular weight fractions of a linear ionic polymer system which does not require the alteration of the structure or chemistry of the ionic polymer and which is adapted for the use of aqueous solvents and elution liquids.